netdev - RE: [PATCH 1/1] bonding: Use notifiers for slave link state detection

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Date:	Wed, 27 Jan 2016 20:00:43 +0000
From:	"Tantilov, Emil S" <emil.s.tantilov@...el.com>
To:	Jay Vosburgh <jay.vosburgh@...onical.com>,
	zhuyj <zyjzyj2000@...il.com>
CC:	"mkubecek@...e.cz" <mkubecek@...e.cz>,
	"vfalico@...il.com" <vfalico@...il.com>,
	"gospo@...ulusnetworks.com" <gospo@...ulusnetworks.com>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	"Shteinbock, Boris (Wind River)" <boris.shteinbock@...driver.com>
Subject: RE: [PATCH 1/1] bonding: Use notifiers for slave link state
 detection

>-----Original Message-----
>From: Jay Vosburgh [mailto:jay.vosburgh@...onical.com]
>Sent: Monday, January 25, 2016 10:01 PM
>To: zhuyj
>Cc: mkubecek@...e.cz; vfalico@...il.com; gospo@...ulusnetworks.com;
>netdev@...r.kernel.org; Shteinbock, Boris (Wind River); Tantilov, Emil S
>Subject: Re: [PATCH 1/1] bonding: Use notifiers for slave link state
>detection
>
>zhuyj <zyjzyj2000@...il.com> wrote:
>
>>On 01/26/2016 08:43 AM, Jay Vosburgh wrote:
>>> <zyjzyj2000@...il.com> wrote:
>>>
>>>> From: Zhu Yanjun <zyjzyj2000@...il.com>
>>>>
>>>> Bonding will utilize notifier callbacks to detect slave
>>>> link state changes. It is intended to be used with miimon
>>>> set to zero, and does not support the updelay or downdelay
>>>> options to bonding.
>>>>
>>>> Because of link flap from the slave interface, if the notifier
>>>> is NETDEV_UP while the actual link state is down, it is not
>>>> necessary to continue.
>>>>
>>>> Signed-off-by: Jay Vosburgh <jay.vosburgh@...onical.com>
>>> 	I haven't signed off on this patch.
>>>
>>> 	I've just started some testing, but as before immediately get an
>>> RCU warning; it looks to be coming from bond_miimon_inspect_slave();
>>>
>>> [  316.473050] bond1: Enslaving eth1 as a backup interface with an up
>link
>>> [  316.473059]
>>> [  316.473806] ===============================
>>> [  316.475630] [ INFO: suspicious RCU usage. ]
>>> [  316.477519] 4.4.0+ #38 Not tainted
>>> [  316.479094] -------------------------------
>>> [  316.480765] drivers/net/bonding/bond_main.c:2024 suspicious
>rcu_dereference_check() usage!
>>>
>>> 	This is presumably because the "case NETDEV_DOWN" call to
>>> bond_miimon_inspect_slave does not hold RCU.  It does hold RTNL, though,
>>> which should be safe for this usage (RTNL mutexes changes to the active
>>> slave).  The appended patch on top of the original makes the warning go
>>> away.
>>>
>>> 	I'm still testing the patch and have no comment about its
>>> functionality as yet.
>>>
>>> diff --git a/drivers/net/bonding/bond_main.c
>b/drivers/net/bonding/bond_main.c
>>> index 9f67948..e3faee9 100644
>>> --- a/drivers/net/bonding/bond_main.c
>>> +++ b/drivers/net/bonding/bond_main.c
>>> @@ -2014,14 +2014,14 @@ static int bond_slave_info_query(struct
>net_device *bond_dev, struct ifslave *in
>>>     /*-------------------------------- Monitoring
>>> -------------------------------*/
>>>   -/* called with rcu_read_lock() */
>>> +/* called with rcu_read_lock() or RTNL */
>>>   static int bond_miimon_inspect_slave(struct bonding *bond, struct
>slave *slave,
>>>   				     unsigned long event)
>>>   {
>>>   	int link_state;
>>>   	bool ignore_updelay;
>>>   -	ignore_updelay = !rcu_dereference(bond->curr_active_slave);
>>> +	ignore_updelay = !rcu_dereference_rtnl(bond->curr_active_slave);
>>
>>Thanks a lot.
>>Because kernel v4.4 needs this kind of patch, I backport this patch from
>>net-next to kernel v4.4.
>>
>>If it is not appropriate, I will revert this patch.
>
>	I don't understand what you mean here.
>
>	I've tested the patch (with my above modification), and while I
>seem to be hitting an unrelated bug in the ARP monitor, I believe this
>patch will misbehave when the ARP monitor is running.
>
>	For example, if arp_interval=1000 and miimon=0, the link state
>notifier callback will change a slave to up should a notifier event take
>place.  So, hypothetically, if a slave is "down" according to the ARP
>monitor (but actually carrier up), and then experience a carrier down
>then up transition, the slave would be set to "up" even though the ARP
>monitor believes it to be down.
>
>	I'm not able to induce the speedy link flap events, so I'm not
>sure about this portion of the patch:
>
>+	/* Because of link flap from the slave interface, it is possilbe that
>+	 * the notifiler is NETDEV_UP while the actual link state is down. If
>+	 * so, it is not necessary to contiune.
>+	 */
>+	switch (event) {
>+	case NETDEV_UP:
>+		if (!link_state)
>+			return 0;
>+		break;
>+
>+	case NETDEV_DOWN:
>+		if (link_state)
>+			return 0;
>+		break;
>+	}
>+
>
>	Unless I misunderstood, Emil's comments elsewhere suggest that
>the current ixgbe driver won't cause those, though.

I ran tests with the above checks and I can't get them to trigger either way.
So at least in my setup this patch has no effect.

Thanks,
Emil